In analytical instruments based on Raman spectroscopy, especially the fiber optic variety (M. M. Carrabba and R. D. Rauh, U.S. Pat. No. 5,112,127), the magnitude of the Raman signal detected depends on, among other things, the power delivered by the excitation light source and the alignment of the optical components within the analyzing system. For quantitative measurements, therefore, a reference must be provided which can account for the inevitable variations in these conditions. In an article by H. Xiao et al. (Applied Spectroscopy 52, 626 (1998)), this was accomplished by depositing a diamond film at the end of the fiber optic probe. The Raman signal from the diamond then served as the reference. But the polycrystalline film drastically reduced the optical power incident on the sample. In a second article by X. Zheng et al. (Applied Spectroscopy 55, 382 (2001)), the diamond reference was incorporated in the form of a small single crystal embedded in a glass tip formed at the end of the fiber-optic probe. However, since the diamond particle had to be slightly offset from the main light path in order to avoid excessive scattering, the ratio of the Raman signals from the diamond and the sample still varied with changes in the optical alignment of the system. In yet another article by R. N. Favors et al. (Review of Scientific Instruments 76, 033108 (2005)), the excitation laser beam was made to form two foci, the first of which being situated inside a reference sapphire plate and the second inside the sample. In this case, because the Raman scattering cross section of sapphire is relatively small, a very long data acquisition time was needed in order to produce a useful reference.
Accordingly, what is needed in the art is a self-referencing Raman probe which will overcome the above mentioned deficiencies.